From the point of view of energy and environmental issues, basic researches on estimation of photovoltaic (PV) power have been carried out in order to solve the problems that occur to a PV-diesel hybrid system on a ship because of irregularity of the PV power. The results include :1.Estimation of PV power (solar irradiance) by image processing of clouds ; First, we have developed a basic method to judge weather conditions, that is "fine" or "cloudy, " in a few minutes by means of detection of clouds motion in the sky that mostly affects the PV power with machine vision. Next, a method to estimate solar irradiation over a few minutes has been considered. The method is based on analysis of the ratio of clouds that is going to cover the sun. Estimation of solar irradiation over a longer time, a few hours, has been investigated by using an all-sky camera, too. Furthermore, we have studied a basic method to discriminate between "thick" and "thin" clouds to estimate transmittance of the sunlight.2.Estimation of PV power (solar irradiance) considering the characteristics of solar irradiance ; We have devised a method to estimate daily solar irradiation by the estimation of the atmospheric transmittance from the theoretical equation of solar irradiance and observed solar irradiance. The experiment confirmed that the method precisely estimated the daily solar irradiation on more than half days in 2000.3.Control of the PV-diesel hybrid generation system ; A basic method to control the PV-diesel hybrid generation system using the short-time and long-time estimation of PV power mentioned above has been proposed.Future works are applied studies to combine above basic studies as a system.